Although hand tools have yielded many functions to machine tools, hand tools remain indispensable in dozens of skilled occupations, the building trades being an example.
An important function of the hand tools known as digging irons is to dig over buried live power lines which would be shorted-out or broken by powered digging implements, and which can be hazardous to workmen in the area.
Sometimes the location and depth of underground power lines may be known exactly, sometimes within a few feet, and sometimes the presence of power lines may be only suspected. Today with all the underground utility lines and more going in daily, it is almost impossible to dig safely.
Voltages commonly encountered in underground power lines range from 220 volts to 7200 volts.
In the prior art digging irons have to, at least some extent, been conductive, either having large metallic structure or else wettable structures with reduced dielectric constant when wet.
Many shock and burn injuries, sometimes fatal, have happened to workmen who encountered undergound power lines while exploring with ordinary digging irons.
The following U.S. patents are representative of known prior art:
No. 2,553,327 issued to W. A. Norman on May 15, 1951, discloses a device which might be used as a digging bar in that it has a metal chisel shaped blade at the bottom, and which has a handle 10 made of wood and set into a metal tubular socket at the bottom end of the handle. The handle ends in an enlarged portion at the top.
No. 2,457,258 issued to G. P. Mitchell on Dec. 28, 1948 for an insulation puller has a "handle 1 preferably of wood or any other insulation material" but has no digging tip;
No. 436,157 issued to C. H. Slocom on Sept. 9, 1890 discloses a tack puller with metal tool portion at bottom and a tack hammer head at the top, presumably of metal also.